AVCompositionTrack

An AVCompositionTrack object provides the low-level representation of tracks a track in an AVComposition object, comprising a media type, a track identifier, and an array of AVCompositionTrackSegment objects, each comprising a URL, and track identifier, and a time mapping.

The timeMapping.target.start of the first track segment in a composition track is kCMTimeZero, and the timeMapping.target.start of each subsequent track segment equals CMTimeRangeGetEnd(<#previousTrackSegment#>.timeMapping.target).